Tnx!
And one more question about aftermarket mids - Does the exhaust smell worse? Like when you drive with open windows in the traffic...

What is the price of n55 mids?
Berk mids are $599 + $50 shipping to Russia from their site, I think that is good prise and they are 2.5" which is not that loud like 3.0" Maddad

There is no way that you will have a gas smell because of a mid pipe change since you still will keep primary cats in your down pipes. No one in my knowledge reported any issue like this with any of the well known aftermarket midpipes.

Price depends on where you order it and from whom. In US it can be as cheap as 300 USD, in Europe can be a bit more than 400 plus shipping. Try not to order from official BMW dealer if you don't want to pay more.